NDIR gas analysis measures the concentration of a gas in a sample by determining the amount of absorption of light which occurs at wavelengths which are normally selected to coincide with a relatively strong absorption band that is characteristic of the gas to be measured. In its simplest form, an NDIR gas analyzer contains a radiation source, an optical interference filter, a sample chamber, a detector and associated electronics. In operation, light is emitted from the radiation source and passed through the sample chamber where a portion of the light is absorbed by a sample gas. Next, light is passed through the filter to remove undesired wavelengths of light and then the remaining filtered light is passed on to the detector which measures the strength of the filtered light. Finally, the associated electronics calculate the concentration of the gas being measured in the sample cell.
The theory of NDIR gas analysis is well established. It has long been considered one of the best methods for gas measurement. However, it is not suitable for many uses because of its complicated and expensive implementation. In designing a low cost NDIR gas analyzer, there are a number of trade offs in the design which must be evaluated and balanced for a particular end use. The optical scheme of the NDIR gas analyzer should be highly efficient and should provide the maximum possible signal on the detector. There should also be an efficient way to exchange gas inside the sample chamber with ambient gas through the diffusive material. However, the diffusive material should have enough density to protect the inside of the sample chamber from particles of dust. As a result, a good design should take the following limitations into account: (1) The density and the thickness of the diffusive material should be efficient to protect against dust and other unwanted particles. (2) The signal to noise ratio on the detector should be sufficient to measure the signals. (3) The power consumption of the source is limited, especially in the case of low power applications powered by a battery. (4) The response time of the sensor. And, of course, cost must be considered in meeting these limitations.
Over the years, various improvements have been made to simplify NDIR gas analyzers in order to reduce the cost of such devices. Examples of some improvements are set forth in U.S. Pat. Nos. 5,163,332, 5,222,389 and 5,340,986, all of which involve diffusion-type NDIR gas analyzers which rely upon a specularly reflective waveguide. Advantages of such devices are simplicity of design and cost. By relying upon diffusion to bring gas into the sample chamber, such devices eliminate the need for more complex and expensive components associated with NDIR gas analyzers which must rely on a pump to create a gas flow into and out of the gas sample chamber. By relying upon a waveguide, such devices use one of the most efficient ways to transport light from the source to the detector through the gas chamber. While such improvements have advanced the state of the art of NDIR gas analyzers, there are still many applications in which NDlR gas analyzers cannot be used when low cost is an integral design constraint, especially when a quick response time is required.
Accordingly, a continuing need exists for inexpensive NDIR gas analyzers. In addition, there is also a continuing need for further improvements in NDIR gas analyzers which will increase their response time in low cost applications.